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An Empirical Model of Glacio-Isostatic Movements and Shore-Level

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An Empirical Model of Glacio-Isostatic Movements and Shore-Level R-01-41 An empirical model of giacio-isostatic movements and shore-level displacement in Fennoscandia Tore Passe Sveriges Geologiska Undersdkning August 2001 Svensk Karnbranslehantering AB Swedish Nuclear Fuel and Waste Management Co Box 5864 SE-102 40 Stockholm Sweden Tel 08-459 84 00 +46 8 459 84 00 Fax 08-661 57 19 +46 8 661 57 19 ISSN 1402-3091 SKB Rapport R-01-41 An empirical model of giacio-isostatic movements and shore-level displacement in Fennoscandia Tore Passe Sveriges Geologiska Undersdkning August 2001 Keywords: giacio-isostatic uplift, shore-level displacement, eustasy. This report concerns a study which was conducted for SKB. The conclusions and viewpoints presented in the report are those of the author(s) and do not necessarily coincide with those of the client. PLEASE NOTE THAT ALL MISSING PAGES ARE SUPPOSED TO BE BLANK Abstract Shore-level displacement in Fennoscandia is mainly due to two co-operative vertical movements, glacio-isostatic uplift and global eustatic sea level rise. The course of the glacio-isostatic uplift has been made discernible according to an investigation of the lake-tilting phenomenon (Passe 1996a, 1998). This information made it possible to start an iteration process that has given mathematical expression for factors involved both within the isostatic movements and the eustatic rise. There are two components involved in glacio-isostatic uplift. The main uplift, still in progress, acts slowly and is thus called the slow component. Arctan functions have proved to be suitable tools for describing the slow component. There are two main factors involved in the function used for calculation; As (m), the download factor (m) and 1 Bs (y ), which is an inertia factor. A strong linear correlation between the inertia factor Bs and lithosphere thickness has been found in the model. There was also a fast component involved in the crustal changes at the end of Late Weichselian and early Holocene. This component gave rise to fast subsidence followed by fast uplift during the final part of the deglaciation. Crustal subsidence is assumed to be due to reloading of the crust in the central parts of Fennoscandia during the Younger Dryas stadial. Normal distribution functions are used for calculating this component. Glacio-isostatic uplift and thus a regressive shore-level displacement was extremely rapid around 10 300 years BP. This fast regression was contemporaneous and occurred in a similar way at the West Coasts of Norway and Sweden as well as in the Baltic. The "drainage" of the Baltic Ice Lake has been interpreted in the model as due to this fast regression. The slow component is most probably due to viscous flow in the asthenosphere and the fast component is assumed to be due to its elasticity. Sammanfattning I området, som täcktes av den skandinaviska isen under den senaste istiden, utgör strandförskjutningen en funktion av två samverkande vertikala rörelser, glacial-isostatisk landhöjning och eustatisk havsytehöjning. Genom sjöstjälpningsmetoden har ett ungefärligt förlopp av landhöjningen kunnat fastställas (Passe 1996a, 1998). Denna kunskap har gjort det möjligt att, i kombination med empirisk information från strandförskjutningskurvor och den nuvarande relativa landhöjningen, beskriva både landhöjningens och havsytans förändringar matematiskt. Beskrivningen visar att landhöjningen styrs av två komponenter, en långsam och en snabb. Den långsamma komponenten utgör den största delen av den glacial-isostatiska landhöjningen och är en pågående rörelse. Denna komponent beskrivs med arctan funktioner. Det långsamma landhöjningsförloppet bestäms främst av två faktorer, 1 As (m), nedtryckningsfaktorn och Bs (y ), som utgörs av en tröghetsfaktor. En stark linjär korrelation har konstaterats mellan tröghetsfaktorn Bs och litosfärens mäktighet. I slutfasen av isavsmältningen, dvs i slutet av senweichsel och under tidig holocen, skedde en kortvarig snabb sänkning av jordskorpan, vilken omdelbart följdes av en lika snabb höjning. Dessa jordskorpsförändringar sammanfattas som den snabba komponenten och beskrivs med normalfördelningsfunktioner. Den kortvariga sänkningen antas bero på en tillväxt av inlandsisen och som följd av denna en förnyad nedpressning under Yngre Dryas stadialen. Landhöjningen och därmed strandförskjutningen var extremt snabb omkring 10 300 år BP. Efter den norska och den svenska västkusten, men också i forna Östersjön, skedde strandförskjutningen med liknande förlopp under denna tid. "Baltiska Issjöns tappning" har tolkats som en snabb regression till följd av den snabba komponenten. Contents 1 Method of the modelling 7 1.1 Objective 7 1.2 Method 7 1.3 Calibration of 14C-values 11 2 Formulae used in the modelling 13 2.1 General 13 2.2 The eustasy 13 2.2.1 Sea level changes 13 2.2.2 Water level changes in the Baltic 15 2.3 Uplift formulae 15 2.3.1 The slow component 15 2.3.2 The fast component 17 3 Results of the modelling 19 3.1 Shore-level curves 19 3.2 Regional result 25 4 Interpretation of the isoline maps 31 4.1 The inertia factor Bs 31 4.2 The pattern of crustal changes 33 4.3 Time dependence of crustal changes 37 4.4 AfandBf 38 5 The Baltic 41 6 Test and improvement of the model 45 7 Summary 47 8 References 51 1 Method of the modelling 1.1 Objective The objective is to find mathematical expressions that describe shore-level displacement and glacio-isostatic uplift in the area covered by Scandinavian ice during the Weichselian glaciation. As the mathematical expressions are based solely on empirical data, they can be used for evaluations of both geological and geophysical parameters involved in the glacio-isostatic process. The model is purely empirical while most other models are based on more or less well-supported assumptions about ice thickness, deglaciation rates and geophysical parameters. The author, in SKB Technical Reports (1996b, 1997), presented two mathematical expressions of shore-level displacement in Fennoscandia. Passe & Andersson (2000) transformed the last model into a GIS application. Earlier works was mainly based on shore-level data from the coastal areas. Utilising information about relative recent uplift from precision levellings extends this third part of the work. Using this information means that the empirical model also includes inland areas. 1.2 Method Shore-level displacement (S m) in Fennoscandia is mainly due to two interactive vertical movements, glacio-isostatic uplift (U m) and global eustatic sea level rise (E m), Figure 1-1. Shore-level displacement is estimated by: S = U - E Equation 1-1 If the eustatic rise of the sea level were known in detail it would have been possible to calculate the glacio-isostatic uplift directly from the shore-level curves. Fairbanks (1989) has published a eustatic curve, which is generally accepted and commonly used in shore-level modelling. As will be demonstrated later (Chapter 2.2) the reliability of this curve is insufficient and therefore cannot be used for accurate shore-level calculations. Physical models of glacio-isostatic uplift have been presented in recent years by e.g. McConnel (1968), Cathles (1975), Peltier (1976, 1988, 1991), Clark et al. (1978), Nakada & Lambeck (1987, 1989), Fjeldskaar & Cathles (1991), Lambeck (1991), Nakiboglu & Lambeck (1991) and Lambeck et al. (1998). The formulae for the course of glacio-isostatic uplift, used in these models, are mainly derived from rheological parameters. However the authors have quite different opinions about these parameters. 200 - 150 S = - E 100 s *~ — ~— — 0- — - -50 / / E ' -100 14000 12000 10000 8000 6000 4000 2000 0 years BP Figure 1-1. Eustasy (E) and crustal uplift (U) determine shore- level displacement (S), by S = U-E. Passe (1990a, 1996a, 1998) investigated glacio-isostatic uplift based on the lake-tilting method. Lake-tilting data show the difference in the course of crustal uplift between two sites, Figure 1-2. The point with empirical lake-tilting investigations is that the course of glacio-isostatic uplift can be expressed in mathematical terms without using rheological assumptions. By magnifying the function, which describes the lake-tilting, it has been possible to start an iteration process to create a mathematical expression of the shore- level displacement. The main input data, besides the lake-tilting information, are shore-level curves from the area covered by Scandinavian ice during the Late Weichselian. The shore level curves are compared to the curves derived from the mathematical expressions. Informa- tion concerning present relative uplift (mm/y), recorded by precision levelling and tide gauge data, has also been used. In Finland records are presented by Kaariainen (1963, 1966), Suutarinen (1983) and Kakkuri (1987), in Sweden by RAK (1971, 1974), in Denmark by Sim onsen (1969) and Andersen et al. (1974), in Norway by Bakkelid (1979). The map of the present relative uplift presented by Kakkuri (1987) is shown in Figure 1-3. Ekman (1996) has compiled information of the present rate of crustal movements in Fennoscandia mainly from the sources mentioned above, Figure 1-4. Recent relative uplift recorded by tide gauge data includes eustatic changes. A eustatic rise in the order of 1 mm/year has been reported by several authors including Lizitzen (1974), Morner (1977, 1980a) and Ekman (1986). Compilations by Emery & Aubrey (1991) and Nakiboglu & Lambeck (1991) indicate a present eustatic rise in the order of 1.2 mm/year. Lambeck et al. (1998) estimates the present rise to 1.05 mm/year. Lake Fegen Depth m -2 -4 -6 -10 -12 16000 14000 12000 10000 8000 6000 4000 2000 cal y BP Figure 1-2. Radiocarbon dates for ancient lake levels in lake Fegen (Passe 1990a, 1996a). The two lowermost points, denoted by red squares, are derived from the gradient of shorelines formed during formation of the Goteborg moraine and the Berghem moraine. The curve shows difference in land uplift between the outlet and the southern part of the lake expressed by an arctan function.
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